Recently, the advent of a practical solar battery excellent in conversion efficiency has been desired. Regarding the conversion efficiency of a P—N junction solar battery, the limiting value under the ideal condition under which the excitation of excitons, which absorbs sun light with a distribution of black-body radiation at a surface temperature of 6000K is balanced with radiation emitted from the deexcitation of excitons obtained as a function of a band gap Vg. The limiting value of the conversion efficiency reaches a maximum value of 31% at Vg=1.3 eV close to the band gap of silicon crystals. The maximum value is considered to be theoretical limit of the conversion efficiency of a silicon solar battery.
The conversion efficiency of the silicon P—N junction solar battery is practically half or less the theoretical limit of the conversion efficiency. Examples of causes of the reduction in conversion efficiency include: the shortened lifetime of carriers at the junction interface between electrodes; and the recombination of carriers in the depletion layer of the P—N junction. The conversion efficiency of the silicon P—N junction solar battery has been improved up to 24% due to the lifetime of carriers being prolonged by passivation of the junction interface between electrodes. Recombination of carriers in the interface is suppressed by inserting, as an intermediate layer of a hetero junction, a layer of amorphous silicon (a-Si:H), which serves as a buffer area, with a large band gap. Thereby, a high open-circuit voltage is obtained and then the conversion efficiency has shown a value exceeding 20%.
Crystalline silicon P—N junction solar batteries based on crystalline silicon substrates have monopolized the market for solar batteries. However, the cost of raw materials has become a greater part of the manufacturing cost as such production has increased. It has been reported that the cost of raw materials made up over 70% of total costs in some cases. A significant reduction in the cost of raw materials for solar batteries comprised of silicon thin films has been attained with no use of a crystalline silicon substrate being needed. Such solar battery, which is flexible and has a large area, is attained by depositing a silicon thin film on a 1 m2 glass substrate, and has been rapidly entering the market in recent years. The introduction of new concept solar cell technologies which can realize an increase in the conversion efficiency to a level so high that it greatly exceeds the theoretical limit of 31% of the silicon P—N junction solar battery is indispensable in the development of photovoltaic technologies aiming to achieve progress in “autonomous” energy supply systems, which lighten the load of electric power supply systems, and, in turn, aiming to expand use of photovoltaic energy supply systems.
Conventionally, a silicon quantum dot solar battery has employed nonocrystalline silicon (nc-Si) that is dispersed in a silicon oxide (SiO2) layer. An nc-Si/SiO2 quantum dot layer is formed by: initially stacking amorphous silicon (a-Si) layers and silicon oxide (SiO2) layers in an alternate manner; and annealing the resultant stack for about one hour at 1050° C. to 1100° C., so as to precipitate silicon in the a-Si layer, which forms nc-Si. Silicon nitride (Si3N4) or silicon carbide (SiC) may be used, instead of the stacked SiO2 layers (see G. Conibeer et al., “Silicon quantum dot nanostructures for tandem photovoltaic cells”, Thin Solid Films, 516 (2008), pp. 6748-6756) (hereinafter Conibeer et al.). In the formed nc-Si/SiO2 quantum dot layer, nc-Si is distributed with a broad particle diameter dispersion ranging from fine particles of 2 nm in diameter to large particles of 5 nm or more in diameter. Furthermore, the particle spacing of nc-Si is not uniform and particles separated by spacing of 5 nm or more can often be found. The same mostly holds true for nc-Si/Si3N4 and nc-Si/SiC quantum dot layers. The density of the silicon quantum dots is in the order of 1010 per unit area (1 cm2) and is up to 1×1011 at most in each layer.
The present inventors have continued the search and development of silicon nanoclusters (see Patent Documents JP 2001-158956 A and JP 2007-162059 A, and Non-Patent Documents Yasusi Iwata et al., “Array order formation of silicon nanoblocks and practical application of thin film deposition system”, Laser Processing Academic Journal, 10 (2003) pp 57-60, and Yasusi Iwata et al, “Narrow size-distributed silicon cluster beam generated using a spatiotemporal confined cluster source”, Chem. Phys. Lett., 358 (2002) 36-42).